Ok but how did you convert 111111111111 in decimal i.e. (2^12 -1)?
And why multiply (2^12 -1) with the step size inorder to find the F.S. output voltage?!
If you read about binary numbers you will find out how you convert it.
If you imagine the output voltage divided into discrete values (steps) and not in a continuous analogue form, and each step equals 10mV then the maximum output voltage is the number of steps times 10mV.